Fix travis macos+scons build.

Steps:
1) added `brew install libffi`, was told it was already installed and to try reinstalling.
2) changed to `brew reinstall libffi`, was told it was "keg only" and was provided the PKG_CONFIG export statement.
3) added export statement, seems to work.

utils/travis/install_deps.sh

@@ -6,6 +6,7 @@ if [[ $TRAVIS_OS_NAME == 'osx' ]]; then
 
 	brew update
 	brew install scons cairo pango moreutils sdl2_image sdl2_ttf sdl2_mixer
+  brew reinstall libffi
 
 else
 

utils/travis/steps/script.sh

@@ -24,6 +24,8 @@ if [ "$TRAVIS_OS_NAME" = "osx" ]; then
         ./utils/travis/check_utf8.sh
         ./utils/travis/utf8_bom_dog.sh
         "$CXX" --version
+        export PKG_CONFIG_PATH="/usr/local/opt/libffi/lib/pkgconfig"
+        
         scons wesnoth wesnothd campaignd boost_unit_tests build=release \
               ctool="$CC" cxxtool="$CXX" cxx_std="$CXXSTD" \
               extra_flags_config="-pipe" extra_flags_release="$EXTRA_FLAGS_RELEASE" strict=true \